This invention relates to methods for making integrated circuits and more particularly methods for making integrated circuits using EUV lithography and the reflective masks therefor.
In making integrated circuits it is desirable to improve the lithography by using shorter and shorter wavelengths for exposing photoresist on semiconductor wafers. These shorter wavelengths allow for higher resolution and require special masks for achieving that higher resolution. One of the techniques that is being developed for high volume manufacturing is using extreme ultraviolet (EUV) frequencies, which have very short wavelengths. Wavelengths from 4 to 25 nanometers (nm) are considered EUV. This technology generally requires a reflective mask, as distinct from a transmission mask, because materials useful as mask materials tend to have high absorption at EUV wavelengths. EUV light is reflected off of the mask and exposes the photoresist according to the pattern of mask. The mask has a reflective portion and an absorber portion so that the reflective pattern is what is actually exposed on the photoresist. In making these masks, the features that are ultimately reflected onto the integrated circuit are extremely fine features. Accordingly, the mask must be extremely high quality and only provide the desired pattern.
Masks without any defects are very difficult to achieve so these masks are made in such a way that they are repairable in order to achieve the actual desired pattern. In order to effectively achieve the repair the mask must be inspected very accurately so that all of the flaws are found. These flaws occur generally in one of two categories. One category is that a portion of an absorber layer is removed where it should be present. Another category is that there is absorber material in areas where it should not be present. In order to locate these two categories of defects, equipment has been developed for this purpose. The equipment images the masks at a wavelength that affects the ability to accurately achieve the desired inspection. A common frequency currently utilized is in the deep ultra-violet (DUV) range of 240 to 260 nanometers, which is more than an order of magnitude greater than the EUV wavelengths that are used for generating the photoresist pattern on the semiconductor wafer. The wavelength utilized for inspection results in limited resolution between the areas where absorber material is intended to be present and where it is not intended to be present. Maximizing the inspection image contrast is critical in being able to be certain that all of the defects are detected.
Another issue with current EUV masks is that there is some absorber leakage that is exacerbated by a buffer layer between the absorber material and the reflective material. This buffer layer is typically a dielectric material that is used as a repair buffer that protects the reflective layer during the repair of the absorber layer. The increased absorber leakage due to this buffer layer results in the boundary between where the photoresist is to be exposed and where it is not to be exposed on the integrated circuit being fuzzier than it would be without the increased absorber leakage.
Accordingly, there is a need for a mask that is more easily inspected by providing a higher contrast between the absorber areas and the non-absorber areas and for a reduction in absorber leakage.